Buying Phones Abroad

I'm going on holiday to America soon and am thinking about buying a mobile phone while I'm over there.

Aside from issues with warranties, are there any issues with buying a phone from the US, ie will it work back in the UK?

I'm leaning towards the Motorola Atrix. However I'll also consider the HTC Evo 3D, HTC Sensation, LG Optimus 3D and Samsung Galaxy S2 if someone knows what these models are called in the States (assuming that they're also available there).

Alternatively are there any high end phones that any would recommend that are available in the USA but not the UK and yet will work in both countries?

All US phone would be locked to US carrier. Make sure you can unlock it before you buy it. They all can be unlocked, but require technical knowledge above average user.
It's highly recommended to have sufficient knowledge to be able to root US phone and load custom ROM. All AT&T and T-Mobile phones will be loaded with proprietary software and locked down.

Atrix is the same in US. I have US version and it works fine here. It's not 4G, but I don't see any difference. It will be locked to ATT, so you have to unlock it or get already unlocked one.

I don't see HTC Evo 3D as being released anywhere yet.

Sensation is not released yet. In US it will be Sensation 4G.

Galaxy S2 is identical around the world. It's a world phone. Not available in US yet. Original galaxy was called Captivate (aka Craptivate) on ATT.

Original Atrix is locked to the point that it will not allow installation of apps from outside sources. Only Android Market apps. MMS and Internet required manual configuration to get working. Had to load custom ROM to use it normally.

All US phone would be locked to US carrier. Make sure you can unlock it before you buy it. They all can be unlocked, but require technical knowledge above average user.
It's highly recommended to have sufficient knowledge to be able to root US phone and load custom ROM. All AT&T and T-Mobile phones will be loaded with proprietary software and locked down.

Atrix is the same in US. I have US version and it works fine here. It's not 4G, but I don't see any difference. It will be locked to ATT, so you have to unlock it or get already unlocked one.

I don't see HTC Evo 3D as being released anywhere yet.

Sensation is not released yet. In US it will be Sensation 4G.

Galaxy S2 is identical around the world. It's a world phone. Not available in US yet. Original galaxy was called Captivate (aka Craptivate) on ATT.

Original Atrix is locked to the point that it will not allow installation of apps from outside sources. Only Android Market apps. MMS and Internet required manual configuration to get working. Had to load custom ROM to use it normally.

When we brought US AT&T Atrix to UK we used unlock code provided by AT&T (business account) to unlock it and use 3 Mobile SIM card. As 3 Mobile does not have any 2G coverage phone was working fine on 3G. I did not pay attention if there is HSDPA/HSUPA coverage. We were able to make/receive phone calls, but no internet/MMS capability. Playing with APN settings did not get us anywhere as we could not delete AT&T presets. Band selection was always set to automatic.
Eventually we gave up and loaded custom ROM from XDA-Devs, which wiped out all AT&T setting and phone started to work normally.

There are two competing technologies being used in US - original CDMA and GSM/WCDMA. Both technologies are CDMA, but implementation is different and they will not work with each other.
Any phone with Verizon, Sprint, Virgin and other MVNO's using Verizon or Sprint network will not work in Europe.
Entire list is here: http://en.wikipedia.org/wiki/List_of_US_MVNO

Only phones for AT&T and T-Mobile might work in Europe (phone calls will work anywhere in the world, but data/3G band are different). UK uses 900MHz and 2100MHz (mostly) bands. US uses 850MHz(AT&T) and 1700MHz(T-Mobile). If phone has 900 and 2100 supported it will work in Europe.

We are going to Gran Canaria in october, and I am thinking about getting an iPhone while I am there. I am on vodafone, and I am aware that the network will change to vodafone ES while there. What are the things to watch for, and any idea on prices please?

We are going to Gran Canaria in october, and I am thinking about getting an iPhone while I am there. I am on vodafone, and I am aware that the network will change to vodafone ES while there. What are the things to watch for, and any idea on prices please?

That iPhone will most likely be locked to Vodafone ES!!!! It won't work with your UK simcard.
So, either you need to get it unlocked in Spain by Vodafone, or you need some 3rd party unlocking service.

If you choose to buy one unlocked in a non-Vf store, try your UK sim to see if it's really sim-free/unlocked.